Previous work in this laboratory has been concerned with the regulation of the activity of tyrosine 3-monooxygenase (TH) in cell suspensions prepared from a transplantable rat pheochromocytoma. Many different stimuli, including 56 mM K ion, cholera toxin, dibutyryl adenosine 3',5'-monophosphate (dibutyryl cAMP), and the divalent cation ionophore, lasalocid, all produce an activation of this enzyme. Cholera toxin leads to a rise in cAMP levels in these cells; it is likely that the activation of TH by cholera toxin is mediated by this rise in cAMP levels. In contrast, 56 mM K ion does not cause a rise in cAMP levels in pheochromocytoma cells. Therefore, it is unlikely that cAMP mediates activation of TM produced by this treatment. The goal of the proposed research is to clarify further the regulation of TH activity in these cells, and to elucidate the mechanisms by which these various stimuli activate this enzyme. Particular attention will be devoted to determining the role of protein phosphorylation in the regulation of TH activity in pheochromocytoma cells. BIBLIOGRAPHIC REFERENCES: Perlman, R.L., The permeability of chromaffin granules to non-electrolytes. Biochemical Pharmacology, 25: 1035-1038, 1976. Chalfie, M., and Perlman, R.L., Regulation of catecholamine biosynthesis in a transplantable rat pheochromocytoma. Journal of Pharmacology and Experimental Therapeutics, in press 1977.